One of the major current challenges to the cable industry lies in improving the behavior and performance of cables under extreme heat conditions, in particular those encountered during the course of a fire. Mainly for safety reasons, it is vital to maximize the capacity of a cable both to retard the propagation of flames, and to resist the fire. Significantly slowing the advance of flames increases the time available for evacuation of sites and/or for using appropriate extinguisher means. Better fire resistance means that the cable can function longer, since it degrades more slowly. A safety cable must also not be dangerous to the environment, i.e. it must not release toxic fumes and/or fumes that are too dense when it is subjected to extreme heat conditions.
Whether electrical or optical, intended for power transport or for data transmission, a cable is constituted in outline by at least one conductor element extending inside at least one insulating element. It should be noted that at least one of the insulating elements may also act as a protective means and/or the cable may further comprise at least one specific protective element, forming a sheath. However, many of the best insulating and/or protective materials used in the cable industry are unfortunately highly flammable. This is particularly the case with polyolefins and their copolymers, such as polyethylene, polypropylene, copolymers of ethylene and vinyl acetate, and copolymers of ethylene and propylene. At all events, in practice, such excessive flammability proves to be completely incompatible with the fire performance requirements mentioned above.
Many methods exist in the cable industry for improving the fire performance of polymers used as insulating and/or sheathing materials.
Until now, the most popular solution has consisted in using halogenated compounds in the form of a halogenated derivative dispersed in a polymer matrix or directly in the form of a halogenated polymer, as is the case with a PVC, for example. However, regulations are now tending towards prohibiting the use of that type of substance, mainly because of their toxicity and their potential corrosivity, whether on manufacture of the material or during its decomposition by fire. This is the case both when the decomposition in question occurs unintentionally during a fire and also when it is intentionally incinerated. Whatever the case, recycling halogenated materials remains a particular problem.
For this reason, more and more non halogenated fire retardant fillers are being used, in particular metallic hydroxides such as aluminum hydroxide or magnesium hydroxide. Unfortunately, that type of technical solution suffers from the disadvantage of requiring large quantities of fillers to be satisfactory, either in terms of flame propagation retarding capacity or of fire resistance. As an example, the metallic hydroxide content can typically be 150 to 200 parts by weight per 100 parts by weight of polymer resin.
However, any bulk incorporation of a filler causes a considerable increase in the viscosity of the material which receives it. This then inevitably generates a substantial reduction in the extrusion rate, and consequently a significant reduction in productivity, which is unfortunately reflected in the cost price of the composite material.
However, independently of this process aspect, non halogenated fire retardant fillers have in any event proved to be intrinsically relatively expensive. And since they have to be used in large quantities, the cost of the materials in which they are dispersed is further increased.